Because I wanted responsive email signatures with dynamic width and media queries. This stuff is supported by most email viewers, and allows for a very professional look. You can get it working with a tweak in mac mail, by going into the actual signature file in the finder and directly editing it- thought the same would be true in this case, and it is!
I found the file.
It's located in: /var/mobile/Containers/Data/Application/[Mail.app random string of characters use Filza to find it normally]/Library/Preferences/com.apple.MailAccount-ExtProperties.plist
If you write a random word in the sig file you want to be altering in Mail/Contacts/Calendars > signatures, look for that word in the property list editor in the file mentioned above and paste in the html (including all the media queries and whatever else all in one file. Javascript wont run though).
Reboot phone,
Done! You are now the proud owner of one of the worlds first responsive signatures sending from iPhone/iPad.